1. Field of the Invention
This invention relates to micromechanical resonator devices and micromechanical devices utilizing same.
2. Background Art
Vibrating mechanical tank components, such as crystal and SAW resonators, are widely used for frequency selection in communication sub-systems because of their high quality factor (Q""s in the tens of thousands) and exceptional stability against thermal variations and aging. In particular, the majority of heterodyning communication transceivers rely heavily upon the high-Q of SAW and bulk acoustic mechanical resonators to achieve adequate frequency selection in their RF and IF filtering stages and to realize the required low phase noise and stability in their local oscillators. In addition, discrete inductors and variable capacitors are used to properly tune and couple the front end sense and power amplifiers, and to implement widely tunable voltage-controlled oscillators. At present, the aforementioned resonators and discrete elements are off-chip components, and so must interface with integrated electronics at the board level, often consuming a sizable portion of the total sub-system area. In this respect, these devices pose an important bottleneck against the ultimate miniaturization and portability of wireless transceivers. For this reason, many research efforts have been focused upon strategies for either miniaturizing these components or eliminating the need for them altogether.
Recent demonstrations of micro-scale high-Q oscillators and mechanical bandpass filters with area dimensions on the order of 30 xcexcmxc3x9720 xcexcm now bring the first of the above strategies closer to reality. Such devices utilize high-Q, on-chip, micromechanical (abbreviated xe2x80x9cxcexcmechanicalxe2x80x9d) resonators constructed in polycrystalline silicon using IC-compatible surface micromachining fabrication techniques, and featuring Q""s of over 80,000 under vacuum and center frequency temperature coefficients in the range of xe2x88x9210 ppm/xc2x0 C. (several times less with nulling techniques). To date, resonators based on freely-supported, vibrating prismatic beams have achieved frequencies of up to 92 MHz. For use in many portable communications applications, however, higher frequencies must be achieved and are thus important to the success of this technology.
Much like the case for transistors, extending the frequency of xcexcmechanical resonators generally entails scaling of resonator dimensions. Some of the previous VHF demonstrations with clampedxe2x80x94clamped boundary conditions actually used submicron dimensions to avoid Q-limiting anchor losses. Unfortunately, smaller size often coincides with smaller power handling and increased susceptibility to environmental effects, such as contamination or thermal fluctuations. Although recently demonstrated freexe2x80x94free beam xcexcmechanical resonators have been able to achieve frequencies up to 92 MHz with Q""s around 8,000 while avoiding submicron dimensions, as shown in U.S. Pat. No. 6,249,073, whether or not they can maintain their size and Q at UHF frequencies has yet to be seen.
An object of the present invention is to provide a micromechanical disk resonator and micromechanical device utilizing same.
In carrying out the above object and other objects of the present invention, a micromechanical resonator device having at least one mode shape is provided. The device includes a substrate and a disk-shaped resonator disposed above the substrate and having at least one nodal point.
The device may include a support structure anchored to the substrate to support the resonator at the at least one nodal point above the substrate. Both the resonator and the support structure are dimensioned and positioned relative to one another so that the resonator is substantially isolated during vibration thereof. Energy losses to the substrate are substantially eliminated and the resonator device is a high-Q resonator device.
The at least one mode shape may include a radial-contour mode shape and/or a flexural mode shape.
The device preferably further includes a drive electrode structure formed on the substrate at a position to allow electrostatic excitation of the resonator so that the resonator is driven in the at least one mode shape and the resonator and the drive electrode structure may define a capacitive gap therebetween.
The drive electrode structure may be disposed about a periphery of the resonator and the at least one mode shape may include a radial-contour mode shape.
The capacitive gap is preferably a sub-micron, lateral, capacitive gap.
The drive electrode structure may include a plurality of split electrodes.
The device may have a single electrode which not only allows electrostatic excitation of the resonator, but also to sense output current based on motion of the resonator. Then the device has two terminals, one on the electrode, one on the resonator, and both are used for both driving and sensing.
The at least one nodal point may correspond to a center of the resonator and the support structure may be a single anchor positioned at the center of the resonator.
The device may further include a sense electrode structure formed on the substrate at a position to sense output current based on motion of the resonator.
The drive electrode structure may include a plurality of separate input drive electrodes and the sense electrode structure may include a plurality of separate output sense electrodes.
The drive electrode structure may be positioned beneath the resonator and the at least one mode shape may include a flexural mode shape.
The device may be diamond-based or silicon-based.
Further in carrying out the above object and other objects of the present invention, a micromechanical device is provided. The device includes a substrate, a disk-shaped input resonator disposed above the substrate and having at least one nodal point, and a disk-shaped output resonator disposed above the substrate and coupled to the input resonator and having at least one nodal point.
The device may also include support structures anchored to the substrate to support the input and output resonators at their respective nodal points above the substrate.
The micromechanical device may be a filter such as a bandpass filter or an integratable filter.
The resonators may be mechanically coupled together or electrically coupled together.
The device may further include a coupling spring for mechanically coupling the resonators together. The coupling spring can be an extension mode spring. Furthermore, the spring can also be flexural, or even combine two different types of modes (e.g., flexural or torsional).
The device may further include a drive electrode structure formed on the substrate at a position to allow electrostatic excitation of the input resonator and a sense electrode structure formed on the substrate at a position to sense output current based on motion of the output resonator.
The micromechanical disk resonators presented here have the potential to extend the frequency of micromechanical devices well into the GHz range, making them viable in all stages of wireless systems (including cellular phones) from the RF front-end down to IF filtering and mixing and enabling a completely integrated, single chip transceiver.
Disk resonators of the present invention have advantages over the freely supported and clampedxe2x80x94clamped beams used in previous HF designs, including the capability to reach UHF frequencies using low numbered mode shapes, typically leading to improved Q. In addition, their larger size at a given frequency improves their power handling capacity, making them more appropriate for RF front-ends where dynamic range is an important parameter. This size also leads to a larger electromechanical coupling area, which improves the series resistance of electrostatically-driven devices. The larger size also makes these devices easier to manufacture repeatedly (i.e., with repeatable frequency, etc.). The larger size also makes disk resonators less susceptible to xe2x80x9cscaling-inducedxe2x80x9d degradation mechanisms, in the manner of contamination or thermal fluctuations.
This work presents an alternative, perhaps superior, design based on the radial contour mode, or areal dilation, of a xcexcmechanical disk structure, and capable of attaining very high frequencies, while retaining relatively large dimensions. Using this disk design, a frequency of 156 MHz has been attained with a Q exceeding 9,400 and with a diameter of 34 xcexcmxe2x80x94substantially larger than the 11.3 xcexcm required to attain only 92 MHz in previous freexe2x80x94free beam resonators.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.